cmd/compile/internal/types2: process errors in src order during testing

Follow-up on https://golang.org/cl/305573.
As a consequence, re-enable test case that caused problems with that CL.

Change-Id: Ibffee3f016f4885a55b8e527a5680dd437322209
Reviewed-on: https://go-review.googlesource.com/c/go/+/307216


Trust: Robert Griesemer <gri@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>